1. Field of the Invention
The present invention relates to a light emitting element and a light emitting device including a light emitting element. In addition, the present invention relates to a method and a material for sealing a light emitting element.
2. Description of the Related Art
Flat panel displays such as liquid crystal panels have been improved, and attempts have been made on improving quality of picture, reducing power consumption, and improving lifetime. In order to utilize the self-emitting ability of electroluminescent elements for practical application of electroluminescent panels (hereinafter referred to as an EL panels) which employs electroluminescent elements (hereinafter referred to as an EL elements) in the pixels, it is desired to realize vivid and bright displays with reduced power consumption. For this purpose, improvement in power efficiency has been investigated by increasing the current-luminance characteristic of materials used in the EL elements. However, there is a limitation on improvement in the power efficiency by the method described above.
The efficiency to extract the light (light extraction efficiency) that is emitted from a light emitting layer of the EL element is only around 20%. The reason of this low light extraction efficiency is that light emitted from the light emitting layer is attenuated since total reflection occurs when the light passes an interface of films having different refractive indexes and that the totally reflected light is absorbed in the EL element. An alternative reason is that the light from the light emitting layer is irradiated through a side surface of the light emitting element, for example, a side surface of a glass substrate.
Reference 1 describes an EL element with improved light extraction efficiency, which was achieved by reducing the amount of total reflection. In Reference 1, by providing a film having dispersed particles over a transparent conductive film to scatter the emitted light, the population of the light, which passes the interface between the transparent conductive film and a low refractive index film, with an incidence angle larger than the critical angle. (Reference 1: Japanese Published Patent Application No. 2004-303724).
The structure of EL panels are classified into a bottom emission structure (lower surface emission structure) and a top emission structure (upper surface emission structure) depending on the direction to which light is extracted. In the bottom emission structure, light is extracted through a substrate over which an EL element is fabricated. In the top emission structure, light is extracted through the upper side of the EL element. Note that the terms “bottom emission structure” and “top emission structure” are often used to refer to the structure of the organic EL panels. However, in this specification, these words are used to classify the structure of a light emitting element or a light emitting device according to not the kind of the light emitting element but the extracting direction of light.
Since the light emission area of the EL element is not strictly limited in the case of the top emission structure compared with the bottom emission structure, the aperture ratio of the active matrix EL panel can be increased by applying the top emission structure. Therefore, in the active matrix EL panels, the top emission structure is advantageous in lowering power consumption and improving quality of the image.